Long-term annual burning of grassland increases CO2 emissions from soils

Abstract

Grasslands have potential to mitigate against climate change because of their large capacity to store soil organic carbon (SOC). However, the long-term impact of grassland management such as burning, which is still common in many areas of theworld, on SOC is still a matter of debate. The objective of this studywas to quantify the longterm effects of annual burning on CO2 output from soils and SOC stocks. The study was performed on a 62 years old field trial comparing annual burning (AB) to no burning associated with tree encroachment (NB), and to annual mowing (AM) with all treatments laid out in randomized block design with three replicates per treatment. CO2 emissions from soil were continuously measured over two years and were correlated to soil chemical and physical properties. AB and AM produced 30 and 34% greater CO2 emissions from soil than NB (1.80 ± 0.13 vs. 2.34±0.18 and 2.41±0.17 g C-CO2m−2 d−1 for NB, AB and AMrespectively). AB and AMalso produced greater CO2 emissions fromsoil and per gramof soil carbon (1.32±0.1 and 1.35±0.1 mgC-CO2 gC−1 d−1, respectively) than NB(1.05±0.07mgC-CO2gC−1 d−1),which corresponded to significant differences of respectively 26% and 29%. Overall, CO2 emissions fromsoil (perm2) significantly increased with soil water content (r=0.72) followed by SOC stocks (r= 0.59), SOC content (r =0.50), soil bulk density (r= 0.49), soil temperature (r=0.47), C:N ratio (r=0.46) and meanweight diameter (r=0.38). These findings suggest that long-termannual burning increases CO2 output from soils. Additional greenhouse gases emissions from burning itself and alternative grassland management techniques were finally discussed.